Juno, all set to explore Jupiter

Anticipation

C Sivaram discusses the Juno spacecraft that was sent to Jupiter earlier this year, its numerous functions and predecessors

The Juno spacecraft arrived at Jupiter on July 5. Juno was launched in August 2011. The five year-long trip to Jupiter involved a time of two years looping through the inner solar system with a slingshot past Earth (October 2013) to increase its speed sufficiently.

While orbiting Jupiter, it would pass close to the surface to escape the lethal radiation belts surrounding the giant planet (far more intense than the Van Allen radiation belt around the Earth). The charged particles from solar flares, cosmic radiation etc., are trapped by Jupiter’s intense magnetic field which is several tens of times that of the Earth, forming these radiation belts.

The Juno mission aims to unravel enigmas beneath the cloud tops of Jupiter, especially features like the great red spot storm (a persistent anticyclonic storm) that has been whirling around for centuries, although now diminishing, how far down do such storms start and what keeps them going. Recent observations suggest that the spot is emitting much heat.

A well-known enigma is that the planet emits nearly three times as much heat as it receives from the Sun. Explanations range from a slow contraction of the planet as a whole, to the gravitational energy (released as heat) when the heavier helium droplets fall through the hydrogen in the deep interior. Several different forms of water ice and other ices are expected to be present in the interior under the condition of high pressures and densities.    

Jupiter’s visible surface consists of dark belts and bands where the atmospheric gases are descending with bright zones of rising gas between. Dark and bright spots are seen near the belts with wisps and other features suggestive of turbulence. Giant aurorae and electrical storms occur in the planet’s turbulent atmosphere. Juno becomes the first probe to orbit Jupiter in nearly 15 years.

The Galileo spacecraft explored Jupiter for eight years until 2003. It took astounding images of many of Jupiter’s moons (The giant planet has well over 60 moons). These images revealed features like a large ocean under the icy crust of the moon Europa (containing more water than Earth) making it as one of the more promising places to look for life outside the Earth, in our solar system. Also, Ganymede, and perhaps Callisto, are likely to have subsurface oceans. A future spacecraft is expected to land on Europa and explore the other moons also.

To protect the planet’s pristine moons from contaminating microbes that might have hitched a ride from Earth, the Galileo spacecraft was sent hurtling into Jupiter’s atmosphere to burn it up. Juno is also planned to meet a similar fiery end (for the same reason), burning up in the giant planet’s atmosphere in early 2018. However, unlike the Galileo probe which studied both Jupiter and its several moons, Juno will focus (during its twenty months in orbit) on Jupiter alone to ascertain what lies beneath the atmosphere’s colourful cloud stripes with its bands of myriad hues.

Before Juno...

Before the Galileo probe, four other spacecrafts had flown past Jupiter. The Pioneer 10, launched in March 1972 was the first mission to photograph Jupiter at close range. Voyager 1 launched in September 1977 flew by Jupiter in March 1979. Voyager 1 was launched two weeks after Voyager 2 (its twin spacecraft), but on a faster trajectory. The Voyagers reached Jupiter in less than two years (unlike Galileo and Juno probes which took five years) because they took advantage of a rare planetary alignment (Jupiter, Saturn, Uranus and Neptune were all lined up on the same side of the Sun) one behind the other, so that from Jupiter a craft could be swung on to Saturn and then further to Uranus etc. This baseball configuration occurs only once in about 180 years. Both the Pioneers and both the Voyagers have now left the solar system.

In 2003, Pioneers’ power source became so weak that their signal could no longer be heard over 12 billion km. Pioneer 10, carried a gold-plated plaque (on its outside) containing digital information about humanity and the Earth just in case it is intercepted by extraterrestrials.” The plaque shows along with scientific notations sketch of a man and woman, the man’s arm raised in greetings.

The package

The Juno spacecraft  is destined to burn up in Jupiter’s atmosphere in 2018. Juno carries a gravity science instrument, a magnetometer, a plasma wave instrument, a microwave radiometer, a Jovian IR auroral mapper, a Juno Camera and a Jovian Auroral Distribution Experiment. Just as physicians often use outward signs to diagnose conditions inside their patients, planetary scientists get beneath the skin of their dynamic subjects by looking at surface appearances. Penetrating deep inside Jupiter’s atmosphere, thick cloud tops keep lower levels hidden.

The behaviour of giant storms at Jupiter’s visible surface reveal vertical wind and temperature structure far below. Fast jet streams blow along the coloured bands circling the Jovian surface. Observing how storms erupt in these jet streams and disturb them enables deeper understanding (literally) of the atmosphere. The fastest jet streams (wind) of Jupiter have speeds upto 200 m/s. Extremely energetic storm plumes extend at least hundred kilometres down vertically from storm base.

Juno Camera will take the images voted on (by amateurs and others) and post it for processing. During each 11- day orbit Juno would return about 40 MB of camera data. In recent years, inexpensive digital camera and access to advanced image processing has enabled amateurs (along with their continuous coverage) with modest telescopes to image planets (like Jupiter) with high resolution. With all of Juno’s instruments going into operation soon, who knows what it will uncover beneath Jupiter’s clouds in the next 20 months!